1. Technical Field
The present invention relates to a method of non-destructively inspecting a welded joint for incomplete penetration using radiation.
2. Description of the Related Art
In a rotor of a steam turbine, in association with the trend of using high temperature steam, a high-temperature section subjected to high temperature steam is made of material different from a low-temperature section subjected to low temperature steam so that each section is made of material appropriate for its environment and the sections are connected to each other by welding. For instance, the high-temperature section is made of high heat resisting steel, whereas the low-temperature section is made of low alloy steel. FIG. 8 illustrates a common structure of a steam turbine. In FIG. 8, the steam turbine 1 is configured such that a plurality of split hollow disks 7 are fitted together between split hollow disks 5 having support shafts 3 and cylindrical ends and that abutment portions of adjacent two of the split hollow disks 5, 7 are connected at a welded joint part W. In this manner, in the steam turbine 1 manufactured by welding the split hollow disks 5, 7 together, it is important to inspect a welding state of the welded joint part W. Further, as illustrated in FIG. 1, the steam turbine 1 has an interior space I formed therein.
FIG. 9A and FIG. 9B illustrate a welding method of related art for the welded joint part W. In FIG. 9A, the base metal 100 and the base metal 200 constitute the hollow disks that are made of different materials and on joint surfaces of the base metals 100, 200, groove parts 102, 202 and abutment faces 104, 204 are respectively formed. As illustrated in FIG. 9A, the groove parts 102, 202 are formed on a side that faces an exterior space O and the abutment faces 104, 204 are formed on a side facing the interior space I. A welding torch 300 is positioned between the groove part 102 and the groove part 202 from the exterior space O.
Next, as illustrated in FIG. 9B, the abutment faces 104, 204 are melted together and also build-up welding is performed on the groove parts 102, 202 one pass at a time using the welding torch 300. After performing the build-up welding, a penetration part M formed between the abutment faces 104, 204 needs to be inspected for lack of penetration. However, the penetration part M cannot be visually checked from the exterior space O. Thus, an inspection hole 106 is formed in the base metal 100 (or the base metal 200) to insert a borescope (not shown) into the interior space I through the inspection hole 106 from the exterior space O in the direction of arrow a. If the penetration part M is accessible, the borescope is inserted toward the penetration part M from the interior space I in the axial direction of the interior space I (direction of arrow b). Then, using the borescope, the penetration part M is visually checked for complete penetration. This visual inspection is preferably performed immediately after welding the first pass (a root pass) to make it easier to repair the part in case that lack of penetration is found.
It is described in JP 09-108883 A to perform the above inspection on the welded joint part of a steam turbine rotor. Further, JP 09-108883 A also describes that an X-ray source is inserted into the interior space I through the inspection hole and then a radiographic test of the welded joint part is carried out to inspect the welding state of the welded joint part. Further, it is described in JP 2010-201507 A to visually monitor the welding part, in the case of welding a rotor of a steam turbine or the like, by means of a video system integrated in a welding torch. Furthermore, JP 2011-177790 A describes that in the case of welding the steam turbine rotor or the like, the joint surface is formed with a groove and an abutment face and the abutment faces have complementary shapes to form a protrusion and a recess for orienting the joint surfaces of two base metal pieces.